ABSTRACTMicroglia actively survey the brain microenvironment and play essential roles in sculpting synaptic connections during brain development. While microglial functions in the adult brain are less clear, activated microglia can closely appose neuronal cell bodies and displace axosomatic presynaptic terminals. Microglia-mediated stripping of presynaptic terminals is considered neuroprotective, but the cellular and molecular mechanisms are poorly defined. Using 3D electron microscopy, we demonstrate that activated microglia displace inhibitory presynaptic terminals from cortical neurons in adult mice. Electrophysiological recordings further establish that the reduction in inhibitory GABAergic synapses increased synchronized firing of cortical neurons in γ-frequency band. Increased neuronal activity results in the calcium-mediated activation of CaM kinase IV, phosphorylation of CREB, increased expression of antiapoptotic and neurotrophic molecules and reduced apoptosis of cortical neurons following injury. These results indicate that activated microglia can protect the adult brain by migrating to inhibitory synapses and displacing them from cortical neurons.

f7: Model of activated microglia-mediated neuroprotection.Activated microglia displace presynaptic GABAergic terminals, which lowers the threshold for firing excitatory synaptic NMDARs. Increased activity of synaptic NMDARs (‘+’) elevates intracellular Ca2+ levels, which leads to activation of ERK1/2 and calmodulin-dependent phosphorylation of CaMKK and CaMKIV. Both activated ERK1/2 and CaMKIV can phosphorylate CREB at Ser133, which subsequently increases the transcription of neuroprotective genes and inhibitors of apoptosis. Activated ERK1/2 also phosphorylates BAD at Ser112, which dissociates from Bcl-2 and renders it active and consequently prevents apoptosis. In addition to CaMKIV, CaMKK can also phosphorylate Akt, which promotes cell survival through downregulation of pro-death signalling molecules. Thus, two molecular mechanisms are involved in the neuroprotection induced by microglial synaptic displacement: increased expression of survival and neurotrophic genes through CREB-mediated transcription and diminished apoptotic signals through phosphorylation of BAD and Akt.

Mentions:
How is microglia-mediated inhibitory synapse reduction neuroprotective? Reduction of inhibitory synaptic input is a documented neuroprotective mechanism due to the subsequent activation of synaptic NMDARs. Synaptic NMDAR activity is essential for neuronal survival and increased resistance to potential neural trauma38. The key to this neuroprotection is the influx of Ca2+ that accompanies synaptic NMDAR activation, which initiates intracellular Ca2+-dependent signalling events that culminate in de novo gene transcription or post-translational activation of antiapoptotic and neuroprotective molecules2739. Pharmacological inhibition of GABAergic receptor activation by bicuculline lowers the threshold of excitatory synaptic NMDAR activity, resulting in increased synaptic NMDAR-dependent Ca2+ transients and neuroprotection19. Displacing of inhibitory presynaptic terminals by activated microglia mimics this bicuculline effect and induces neuroprotection by a similar mechanism. Essential aspects of the signalling cascades are summarized in Fig. 7.

f7: Model of activated microglia-mediated neuroprotection.Activated microglia displace presynaptic GABAergic terminals, which lowers the threshold for firing excitatory synaptic NMDARs. Increased activity of synaptic NMDARs (‘+’) elevates intracellular Ca2+ levels, which leads to activation of ERK1/2 and calmodulin-dependent phosphorylation of CaMKK and CaMKIV. Both activated ERK1/2 and CaMKIV can phosphorylate CREB at Ser133, which subsequently increases the transcription of neuroprotective genes and inhibitors of apoptosis. Activated ERK1/2 also phosphorylates BAD at Ser112, which dissociates from Bcl-2 and renders it active and consequently prevents apoptosis. In addition to CaMKIV, CaMKK can also phosphorylate Akt, which promotes cell survival through downregulation of pro-death signalling molecules. Thus, two molecular mechanisms are involved in the neuroprotection induced by microglial synaptic displacement: increased expression of survival and neurotrophic genes through CREB-mediated transcription and diminished apoptotic signals through phosphorylation of BAD and Akt.

Mentions:
How is microglia-mediated inhibitory synapse reduction neuroprotective? Reduction of inhibitory synaptic input is a documented neuroprotective mechanism due to the subsequent activation of synaptic NMDARs. Synaptic NMDAR activity is essential for neuronal survival and increased resistance to potential neural trauma38. The key to this neuroprotection is the influx of Ca2+ that accompanies synaptic NMDAR activation, which initiates intracellular Ca2+-dependent signalling events that culminate in de novo gene transcription or post-translational activation of antiapoptotic and neuroprotective molecules2739. Pharmacological inhibition of GABAergic receptor activation by bicuculline lowers the threshold of excitatory synaptic NMDAR activity, resulting in increased synaptic NMDAR-dependent Ca2+ transients and neuroprotection19. Displacing of inhibitory presynaptic terminals by activated microglia mimics this bicuculline effect and induces neuroprotection by a similar mechanism. Essential aspects of the signalling cascades are summarized in Fig. 7.

Bottom Line:
Electrophysiological recordings further establish that the reduction in inhibitory GABAergic synapses increased synchronized firing of cortical neurons in γ-frequency band.Increased neuronal activity results in the calcium-mediated activation of CaM kinase IV, phosphorylation of CREB, increased expression of antiapoptotic and neurotrophic molecules and reduced apoptosis of cortical neurons following injury.These results indicate that activated microglia can protect the adult brain by migrating to inhibitory synapses and displacing them from cortical neurons.

ABSTRACTMicroglia actively survey the brain microenvironment and play essential roles in sculpting synaptic connections during brain development. While microglial functions in the adult brain are less clear, activated microglia can closely appose neuronal cell bodies and displace axosomatic presynaptic terminals. Microglia-mediated stripping of presynaptic terminals is considered neuroprotective, but the cellular and molecular mechanisms are poorly defined. Using 3D electron microscopy, we demonstrate that activated microglia displace inhibitory presynaptic terminals from cortical neurons in adult mice. Electrophysiological recordings further establish that the reduction in inhibitory GABAergic synapses increased synchronized firing of cortical neurons in γ-frequency band. Increased neuronal activity results in the calcium-mediated activation of CaM kinase IV, phosphorylation of CREB, increased expression of antiapoptotic and neurotrophic molecules and reduced apoptosis of cortical neurons following injury. These results indicate that activated microglia can protect the adult brain by migrating to inhibitory synapses and displacing them from cortical neurons.